Bridge

The objective of the event Bridge Building is to build a bridge which is very lightweight and capable of holding a designated weight.

Bridge Building Tips

Getting started: If you know a structural engineer, you should probably ask him/her for advice. It gives you an advantage over less fortunate people like me. (Don't have a structural engineer draw out the entire design! The end does not justify the means!) It is a good idea to be familiar with some of the basic concepts of bridge design, but I recommend that you do not spend too much time researching and planning. You should begin construction as soon as possible so that you will be able to build more bridges and, in doing so, learn from trial and error. Of course, the first step for construction is to drawn up a plan for your bridge. Draw it the exact size you want your bridge, place on cardboard or foam, and cover it with wax paper; you will need to use this to aid in constuction. (As far as I know, but I could be wrong, one cannot build a decent bridge without a plan on which to lay out the pieces.) You next need to buy materials with which to construct your bridge. The rules state the bridge must be constructed only of wood joined by glue. For wood, you are best off using balsa and perhaps a little bass for pieces which undergo a lot of stress. Not all balsa is the same; some pieces are stronger than others. Be sure to feel the pieces before selecting the ones you wish to purchase. For glue, you should buy the fast-drying stuff with the purple lid (whatever it's called) and wood glue. Wood glue is nice when gluing together long pieces (i.e. if you want to layer and arch or i-beam.) Spray is available to make glue set up more quickly; however, the bond will not be as strong. Try to avoid spray. (Be careful! You are likely to constantly glue your fingers to the bridge. You might not have finger prints after working on this event for awhile. I even got glue on my tongue a couple times when I became impatient and tried to bite the glue of my finger before it was quite dry.) Try to be exact when cutting the pieces. I used a band saw and then sanded the ends to the correct angle.

Construction Methods

• Truss: Truss construction, extremely common in structural engineering, is basically construction using a bunch of triangles. The triangle is a very stable, sturdy shape. No matter what frame shape you choose, the cross members of the sides of your bridge should be arranged in a triangular pattern.

• Arch: An arch frame is very sturdy, efficiently transferring pressure to ends of the bridge. Arches can be made from thin pieces of wood by soaking it in water. Construct an arch-shaped jig to hold the wood until it is dry.

• I-beam: I-beams can support more weight than a solid piece of wood. Consider using them for pieces of your bridge under the most stress. Long i-beams can be difficult to build while keeping the top and bottom pieces straight and even. Here's my method: Apply glue down the length of the wood and clamp in place until dry.

An I-beam

Testing

Here's the exciting part. If you don't jump when you here wood crack now, you will for awhile after you've done bridge. Aside from courage, testing doesn't really take much special skill. Just make sure the bucket doesn't swing and avoid piling the sand on one side or the other. Have someone watching to see where the bridge breaks first. Once the bridge breaks, examine it. Determine what parts must be strengthened and which ones you might be able to replace with weaker (thinner, lighter) pieces or eliminate. This is the best method of learning!